The present invention relates to a method and to an apparatus for measuring optical properties of optical devices, particularly spherical specular surfaces and lenses.
The most accurate existing method for testing spherical specular surfaces is illustrated in FIG. 1. Thus, as shown in FIG. 1, a standard interferometric setup, generally designated 2, is used to focus a coherent beam onto the optical surface 3 to be measured. As known, interferometer 2 produces a reference beam and return beam, and measures the phase difference between the two beams. The object 3 can be translated along the optical axis until the interferogram observed on the screen of the interferometer indicates that the beam reflected off the object surface is in phase with the beam reflected off a reference surface, usually referred to as a reference sphere as indicated at 4 in FIG. 1. The optical surface 3 to be measured is first located at the broken line position 3' in FIG. 1 wherein the forward and return beams are in phase, and is then moved, e.g., along an optical rail or table, to the full-line position in FIG. 3 wherein the two beams are again in phase. The distance between these two positions on the optical axis is the radius of curvature of the tested object. Imperfections on the surface of the tested object can be measured from the shape of the distored fringes they produce on the screen in the interferometer 2 when the tested object is in the full-line position of FIG. 1. Thus, the amount of local fringe shifts indicates the surface figure, namely the difference between the actual surface and an ideal sphere measured in waves of the coherent light used by the interferometer.
FIG. 1a illustrates how the existing method is used for measuring concave surfaces, and FIG. 1b illustrates how it is used for measuring convex surface. Thus, when measuring concave surfaces (FIG. 1a), the concave surface, after zeroing the system, is moved away from the interferometer, whereas when measuring convex surfaces it is moved towards the interferometer.
FIG. 2 illustrates the same prior art technique used for testing lenses, as indicated at 5. In this case, a flat mirror 6 is placed in front of the lens 5 to be tested, i.e., away from the interferometer 2. The lens to be tested is first located in the broken-line position illustrated in FIG. 2, corresponding to that of the beam reflected off the back vertex of the lens, and is then moved to the full-line position in FIG. 2 wherein the back focus of the lens coincides with the focusing of the incoming beam. In the latter position, the beam exiting the lens 5 towards the flat mirror 6 is collimated, so that the return beam traces itself exactly to the focal point and into the interferometer 2. The distance between the broken-line and full-line positions of the lens equals the back focal length (BFL) of the test lens 5. Once again, fringe distortions may be used to calculate aberrations and power variations across the lens. Both negative and positive lenses can be tested. Thus, when testing negative lenses, the lens is moved from its zero position away from the interferometer, and when testing positive lenses, it is moved towards the interferometer.
The above traditional method is very accurate, and with proper fringe analysis technology the radius of curvature can be measured to better than 0.1 micrometer. When this resolution is not required, the reflected beam may be analyzed by other techniques for testing beam collimation. Possible alternatives are shearing interferometers, schlieren and other spatial filtering techniques, shadowgraphy, etc. These instruments do not require a reference beam and thus are much less susceptible to noise and vibration as compared to interferometers.
However, the above traditional method suffers from several drawbacks:
1. The range of radii of curvature or focal lengths is limited. When measuring convex surfaces or negative lenses, the object can only be translated towards the converging lens (FIGS. 1 and 2) until they touch. This limits the radius or focal lengths to values smaller than the focal length of the converging lens. Likewise, positive lenses and mirrors cannot be translated beyond the edge of the optical rail or table.
2. The magnification expressed, for example in mm's on the screen to mm's on the object, is not constant but depends on the distance travelled (see FIG. 1).
3. Accurate analysis of the test surface requires proper imaging of the surface onto the observation screen. Failure to do so may introduce undesirable effects, such as diffraction and geometric distortion (namely, an image point may not correspond exactly to its scaled point on the object). For a given optical setup of the detection system, only one position of the object on the rail is optically conjugate to the screen. Objects with different focal lengths are placed in different positions, so that they can all be imaged only if the detection system provides for focusing hardware.
Drawbacks 2 and 3 above may be overcome by recalculating the scale and carefully focusing on each individual object. However, this solution is not practical when fast operation, such as on the production line, is desired.